It is common practice to build fireplaces in homes and build concrete hearths in association with such fireplaces. A concrete masonry unit (CMU) is first constructed utilizing cinder blocks which are mortared into a desired peripheral configuration and the interior thereof is generally filled with brick trash, masonry fill, etc. The concrete masonry unit is built in a floor opening defined by double floor joists. Building codes require that the double floor joists be spaced from the exterior surfaces of the concrete masonry unit to prevent heat from a fire in the eventual completed fireplace from actually burning the floor joists, associated underflooring, debris, etc., and burning down the associated building. At this point in the construction of the fireplace hearth, the concrete masonry unit is spaced from the floor joists by whatever distance is prescribed by the local code, but a form must be built to allow the pouring of the concrete hearth. Conventionally the hearth form is built from wood, appropriately nailed and supported in an area between what will be the front of the fireplace between a front of the concrete masonry unit and the adjacent double floor joists. This wooden hearth form is not, of course, built with any degree of accuracy or reliability and basically serves as a temporary support for the eventually poured concrete. If the carpenter does not build the wooden hearth form correctly and accurately, the spacing eventually created upon the setting of the concrete and removal of the wooden hearth form might not meet the code which in turn would require the concrete hearth to be destroyed, reformed and rebuilt. If the wooden hearth form has gaps or spaces that are not filled by the poured concrete, these can conduct heat outwardly toward the double floor joists, the underfloor, and any scrap materials in the area creating a fire hazard. Likewise, the wooden hearth form must be removed because it is itself a fire danger because of its natural combustibility, and if all or part thereof is not removed, the danger of unintended fire-damage is increased. From the foregoing, such conventional on-site built wooden hearth forms are unreliable. unsafe and costly.